I think one point you're missing is that the vast majority of people use Pinterest for reasons that are different than a social network. Even if it may have some of the same surface-level mechanisms (like a personalized feed or share buttons), it is very much _not_ used as a tool to broadcast personal updates, read news, or cultivate relationships among people. The average user treats it much more like a visual search engine with a bookmarking system built into it than a social network.

Also, this is a minor quibble, but Pinterest does not have a Like system any longer. I know because I worked on deprecating it.